1. Field of the Invention
The present invention relates to a curable resin composition and a color filter. In particular, the present invention relates to a curable resin composition to be used as a protective film of a color filter, called an overcoat (OC) film, which is an essential element for coloring a liquid crystal display device, and a color filter comprising such an overcoat.
2. Description of the Related Art
Hitherto, a liquid crystal display device (LCD) has been used in portable television sets. Over the years, the number of color LCDs having a large size and a large face plane used for office automation equipment and information-related equipment have rapidly increased. An LCD having an image quality which is substantially comparable with a cathode ray tube (CRT) has been commercially available as the result of development of a passive-addressing mode element utilizing a twisted nematic (TN)liquid crystal or a super twisted nematic (STN) liquid crystal and an active-addressing mode element represented by a thin film transistor (TFT).
Many investigations have been made for the improvement of the image quality of a color LCD and its production. Key factors included improvement of quality of a color filter which is essential to coloring of the LCD and a stable supply thereof.
The OC film used on the color filter of the color LCD is required to have the following properties:
(1) The OC film fills a step been pixels to achieve surface flatness. Unevenness of the surface is from 0.1 to 0.2 .mu.m for TN and TFT, and is less than 0.05 .mu.m for STN. PA1 (2) The OC film is resistant to conditions in post-processing at high temperatures such as ITO (indium-tin oxide) sputtering or lamination of panels. PA1 (3) The OC film increases solvent resistance of the color filter. PA1 (4) The OC film has good adhesive properties. When the adhesiveness between the color filter and the OC film is insufficient, failure mode is generated in a reliability test of the LCD, so that the LCD cannot be practically used. For use with the STN element, when the adhesiveness is decreased in the etching process for ITO patterning, the layers are delaminated between the ITO film and the OC film or between the OC film and the color filter. PA1 (5) The OC film is transparent in the visible light range. PA1 (6) The coating liquid for the OC film has good storage stability.
Hitherto, as a material of the OC film for the color filter of an LCD, an epoxy, polyimide or acryl base resin composition has been studied.
However, a conventional resin composition cannot satisfy all of the above requirements at a satisfactory level.
For example, when an ITO film is formed on a conventional OC film by sputtering at a high temperature, minute wrinkles form, the film is discolored to impair the transparency, film thickness is decreased, or the ITO film is minutely cracked, so that the intended color filter may not be obtained.
In the case of the color filter of the STN element, since patterning of the ITO film is necessary after the formation of the ITO film, it is subjected to the post-processing such as photolithography, etching, and so on. In the post-processing, because of insufficient adhesiveness of the OC film, the delamination may appear between the ITO film and the OC film or between the OC film and the color filter. Further, in the heat treatment step when the LCD panel is assembled using the color filter, because of insufficient heat resistance, the OC film is discolored, so that the quality of the whole LCD may be seriously deteriorated.
Since the coating liquid for the formation of the OC film has poor storage stability, two components, that is the base component and the curing agent should be mixed just before coating, or it is troublesome to remove the coating liquid from a coater after coating with a spin coater, etc.